The present invention relates to an electric coil with an overlying vitrified glass winding and method of making the coil and particularly relates to linear variable differential transformers, electric coils, e.g., for use in environments where temperatures range between very high and cryogenic temperature extremes and intense nuclear radiation is extant, and methods of fabricating the coils.
Currently, electric coils, e.g., linear variable differential transformers (LVDT) have been manufactured for use under extreme environmental conditions, for example, in temperature conditions ranging from liquid nitrogen to 650.degree. C. and radiation levels represented by a total integrated flux of 3.times.10.sup.20 NvT. These devices are fabricated of inorganic materials, i.e., ceramics and metals, to avoid the degradation that organic materials experience from high temperature and radiation. Recent refinements in materials and manufacturing technologies have resulted in acceptable products for most applications in these extreme environmental conditions.
However, most such devices have only a very limited life cycle, particularly when subjected to rapid and frequent temperature cycling, combined with severe vibration and shock. Current practice involves winding the coils of the LVDT with a precious metal alloy magnet wire insulated with ceramic insulation. The wire is then cured and heated to a temperature sufficient to drive off the organic binder and cause the ceramic particles of insulation to bond. The magnet wire itself is held in place by application of ceramic cement. The coil thus fabricated is vulnerable, however, to humidity and other moisture. It is also highly fragile due to its relatively low strength and brittleness. Conventionally, such LVDTs are protected by welding them into hermetically sealed housings.
Recent development work in this technology has focussed on coating the cured magnet wire with a first material, i.e., a clay, to overcoat the magnet wire with an insulating ceramic coating free of lead or lead oxide to prevent interaction of the coating and the metal of the magnet wire. A second coating is also applied over the first coating and is intended, upon curing, to produce a moisture-resistant coating. While these techniques can be utilized, the fabricating process requires substantial attention and time because three steps of temperature processing are required, necessitating three controlled heating and cooling steps. The resulting product, however, suffers additionally because of potential spontaneous fracture due to stress created by the difference in coefficients of expansion of the various materials.
According to the present invention, there is provided a coil, for example, a linear variable differential transformer, and a process for fabricating the coil wherein the coil itself is protected against moisture, resists vibration and shock and is more dimensionally compact. Additionally, the process for fabricating the coil comprises a one-step heating process in lieu of multiple heating steps and other processing used previously. Particularly, the coil hereof includes a winding formed of electrically conductive material, such as palladium-silver, with a ceramic insulating material thereabout. One or more layers of glass thread, each comprised of a plurality of spirally wound glass fibers, are wound about the layer or layers of the coil winding. By heating the wound coil, its component parts pass through temperature stages which first enable the organic vapors of the ceramic insulating material on the electrically conductive winding to be released and infiltrate out through the wound glass thread, i.e., between its windings. When the ceramic insulation material has been cured, the coil is further heated to a predetermined temperature corresponding generally to the vitrification temperature of the glass thread. The temperature is held at the vitrification temperature until a portion of the glass thread becomes vitrified as a common mass, while another portion of the glass thread remains as discrete, well-defined, non-vitrified fibers. After cooling, the non-vitrified fibers in the mass of vitrified fibers reinforce the latter about the conductive winding.
More particularly, by properly controlling the temperature and time extant at such temperature, portions of the glass fibers may vitrify, while others remain non-vitrified, forming, in essence, non-vitrified discrete reinforcing fibers for the mass of vitrified fibers. It will be appreciated, of course, that the winding of electrical conductive material may comprise one or more layers, while, similarly, the overlay of glass threads may likewise comprise one or more layers. Additionally, a second or secondary winding of electrically conductive material may be provided about the first layer of glass threads and the underlying first or primary electrical windings, with a second layer of glass threads wound about the secondary layer of electrically conductive material. Thus, two layers of glass threads, each being partially vitrified and having no-vitrified discrete well-defined glass fibers extending through and reinforcing the vitrified glass fibers, may overlie the discrete windings of electrically conductive material.
In another embodiment of the present invention, the fibers of the glass thread may have different vitrification temperatures. In this embodiment, the temperature of the coil may be raised during fabrication to the vitrification temperature of one of the fibers, while remaining below the vitrification temperature of the other fiber or fibers in the glass threads. Thus, the glass fiber having the lower vitrification temperature vitrifies while the remaining glass fibers remain non-vitrified, forming discrete well-defined reinforcing fibers extending through the mass of vitrified fibers.
According to the present process, the winding is thus covered by a partially vitrified glass thread winding which protects against ingress of moisture and resists vibration and shock because of the reinforcing capacity of the non-vitrified glass thread portion or fibers thereof. Additionally, the cost of manufacture is substantially reduced because only a single heat treatment processing step is needed, rather than multiple steps at different temperatures. Further, the layers of glass thread afford a very thin coating about the windings and hence form a compact coil.
While the present invention is particularly useful for linear variable differential transformers, particularly those which are subjected to wide cyclical temperature changes and radiation, the process is equally applicable to coil devices which require a combination of protection against moisture, high temperature, performance and radiation resistance such as inductive half bridge coils, ordinary inductors or transformers, toroids or the like.
In a preferred embodiment according to the present invention, there is provided an electrical coil comprising a winding of at least one layer of electrically conductive material, at least one layer of a glass thread wound about the one layer of electrically conductive material and a first portion of the one layer of glass thread being vitrified and a second portion thereof being non-vitrified to reinforce the vitrified glass layer portion.
In a further preferred embodiment according to the present invention, there is provided a method of forming an electrical coil comprising the steps of winding at least one layer of electrically conductive material about a form, winding at least one layer of a glass thread about the one layer of electrically conductive material, controllably heating the coil to a predetermined temperature such that a first portion of the glass thread is vitrified and retaining a second portion of the glass thread in a non-vitrified state whereby, after heating the coil, the non-vitrified portion reinforces the vitrified portion.
Accordingly, it is a primary object of the present invention to provide a novel and improved electrical coil and method of fabrication thereof which enables a coil for use in environments subject to wide cyclical temperature changes and temperature extremes, as well as radiation, the coil having substantial shock, vibration and moisture resistance characteristics and a complete seal thereabout and wherein the process requires only a single heat treatment step.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.